Optical Properties Of BeO And LiYF₄ Crystals Under High Pressure

In the field of dynamic high-pressure,experiments such as the shock-temperature measurements of non-transparent materials often need to use transparent window materials that match the shock-impedance of the experimental sample(shock-impedance Z=?*D,?is the density,D is the shock-wave velocity).However,the current window materials that can be selected are still very limited,and the shock-impedance they can provide is limited and discrete.Therefore,the development of window materials to make the shock-impedance continuous and cover as large as possible is a research hotspot.Therefore,this paper adopts the method of first-principles calculations to mainly study the optical properties of BeO and LiYF₄crystals under high pressure.In order to explore whether they may become transparent windows for the dynamic high-pressure experiments,and the adaptable pressure and wavelength ranges in these experiments,the possible influencing factors,such as pressure,phase transition and vacancy defects,were considered.Gained some understanding as follows:(1)The absorptive data of the perfect crystal of BeO shows that in the pressure range of 150 GPa,there is no light absorption in the visible wavelength,and as the pressure increases,it leads to a blue-shift in its absorption edge,the transparent region is widened.In addition,the high-pressure structure phase transitions lead to a weak red-shift on the absorption edge,and the oxygen vacancy will lead to a new absorption in the 100-200 nm,but the appearance of this new absorption did not make that there is a light absorption in visible region(wavelength that the dynamic high-pressure experiments focus on).Even considering the influence of factors such as pressure,phase transitions and vacancy defects,BeO crystals can still maintain good transparency in the visible light region.The calculated results of the energy band structure and the density of states are consistent with the absorption spectrums.The band gap width of BeO crystal is large enough under high pressure,and it will widen with the increase of pressure,so even if the oxygen vacancy defect cause the defect energy level,it will not affect its transparency in the visible light region.The calculated data shows that BeO crystal may become a window for shock experiments,and it is recommended to carry out further experimental investigations.In addition,the calculated data in this paper show that the increase in pressure will weakly decrease the refractive index of BeO crystal at the wavelength of 532 nm,while the structural phase transition and oxygen vacancy defects will cause the refractive index to increase significantly.These results are useful for future experimental studies.(2)According to the current situation,the optical property data of the vacancy point defect crystal can explain the shock-experiments'observation well,so the effect of the vacancy point defect on the optical properties of the material under high pressure is very important.The calculated absorption spectrum data of LiYF₄ crystals in this paper show that the existence of vacant defects does not cause new light absorption in the absorption spectrum,does not lead to a red-shift of the absorption edge significantly,that is,it does not significantly reduce its transparency under high pressure.According to the perfect crystal data,in the range of 100 GPa,the light absorption of LiYF₄ crystal only exists in the range of 200 nm,and there is no light absorption in the visible light region(transparent in the visible light region).The data of the reflection spectrum and the energy loss spectrum are consistent with the above results.The two spectrums are very low and flat in the visible and infrared regions,and the effect of vacancy point defects only exists in the ultraviolet region.In summary,LiYF₄ crystals have good transparency in the visible light region,Moreover,its initial density is higher than that of common optical window materials,so it has the potential to be applied to experimental samples with higher shock-impedance.And the refractive index will provide references for future high-pressure experiments.